Previously, we have reported details of the calcium-induced change in the structure of the regulated thin filament as calculated from X-ray diffraction data from skinnned, non-overlapped rabbit psoas muscle fibres (Poole etal Biophys. J. 66, A347, 1994; Holmes; Poole; Lorenz, Biophys. Discussions, Oct. 94). The starting model was based on an atomic model of actin-tropomyosin refined against oriented gel data (Lorenz etal J. Mol. Biol. in press). We conclude that when Ca++ binds tropomyosin must undergo an azimuthal rotation of ca. 30' around the filament from an off-state position in which it lies directly over the myosin binding site, to a position in which it would still impinge on the docking of the tip of the upper-50K domain of the myosin head in the actomyosin model of Rayment etal, Science, 261: 58-65, 1993. The cross-bridge would have to push the tropomyosin a little further round to achieve full docking. These structural findings are entirely consistent with the biochemical data on the regulation of cross-bridge binding which show that all binding is blocked in the Ca++- free state (at least at physiological ionic strengths), and that Ca++ opens the actin site up for weak binding but that the level of strong binding depends on the head concentration and shows cooperatively (McKillop & Geeves, Biophys. J. 65: 693-701). The titration curve of the structural change of the thin filament against Ca++ concentration in the absence of cross-bridge shows little cooperativity and has a lower Ca++ sensitivity than force production in these fibres. We have performed similar structural titrations in the presence of various levels of interacting cross-bridges and show that both Ca++ sensitivity and cooperatively are enhanced, and conclude that the regulatory state of the thin filament is determined by an interplay of Ca++ and cross-bridge binding.